Fentanyl is a synthetic narcotic analgesic commonly used in surgical anesthesia. Most pharmacokinetic studies of fentanyl have been done in relatively healthy surgical patients and normal volunteers. Most of the pharmacokinetic parameters have been determined using a radioimmunoassay, which has been shown to be relatively non-specific in that it does not distinguish between fentanyl and its metabolites. Fentanyl has gained popularity as an agent for analgesia and sedation in critically ill patients. Little is known about how critically ill patients metabolize and eliminate fentanyl. Several gas-liquid chromatographic assays have been developed to analyze fentanyl and its metabolites. These assays have been shown to be highly sensitive and specific for fentanyl and its metabolites. The current study is designed to determine pharmacokinetic parameters of fentanyl in critically ill patients using a highly sensitive, specific, and reproducible gas-liquid chromatographic assay. A total of twenty patients who are receiving fentanyl for clinical purposes are to have frequent serial sampling of blood and urine for assaying.